This invention relates in general to brake systems for motor vehicles, and in particular to fluid pressure operated brake boosters for vehicle braking systems.
Conventional vehicle brake systems are constructed such that the braking force developed by the system is generally proportional to the force applied by the vehicle operator on a vehicle brake pedal. The pedal is linked to a piston in a master cylinder which moves to pressurize the fluid of the brake system, and thus actuate the individual wheel brakes. Generally, in order to provide a sufficiently high pressure to operate the wheel brakes without requiring an excessive effort by the operator, most vehicles include a "boosted" power brake system wherein the force applied to the brake pedal by the operator is amplified or boosted before being applied to the master cylinder. In automobiles and light trucks, this is typically accomplished by incorporating either a vacuum or hydraulically operated boost piston assembly between the brake pedal and the master cylinder.
Vacuum operated boost piston assemblies utilize the pressure differences between the atmosphere and the vehicle's engine vacuum to develop a force across a boost piston or diaphragm. The force developed by this differential pressure across the boost piston may be many times that which could be developed by the operator by pressing against the pedal, and is applied to the piston of the master cylinder to actuate the brakes.
Hydraulic boost piston assemblies utilize the pressure developed by a hydraulic system, for example, using the vehicle's power steering pump, to apply a force to a boost piston. The force applied by the hydraulic system to the boost piston, like the vacuum boost system, may be many times that which could be applied by the operator of the vehicle when operating the brake pedal through the same stroke length used to actuate the hydraulic boost piston assembly. Thus, the increased force applied to the piston of the master cylinder generates higher braking system operating pressures.
Hydraulic brake boosters may be classified into two categories, namely, two-sided piston designs and single-sided piston designs. A two-sided piston design, as used herein, means a brake booster in which one side of the boost piston is selectively exposed to a boost pressure from upstream of a boost pressure control valve, and the other side of the boost piston is continuously connected to the discharge side of the boost pressure control valve, so that a differential pressure developed across the boost pressure control valve may be applied to the boost piston of the brake booster. In contrast, a single-sided piston design, as used in this application, means a brake booster in which a boost pressure is selectively applied to one side of the boost piston of a brake booster, while the other side of the boost piston is vented to a location other than the discharge of the boost pressure control valve, such as a hydraulic system reservoir, or to atmosphere.
Although past hydraulic brake boosters have served relatively satisfactorily, there is a need for hydraulic brake boosters of different designs to increase flexibility in the design of braking system. In particular, there is a need for hydraulic brake boosters which can be used with an associated electrically controlled pressure control valve and which do not require a two-sided piston design.